A wide variety of spinal fixation systems exist. Some systems and their components will be discussed below, and are well known to those skilled in the art of orthopedics. These various systems are meant to help safely secure and stabilize the spine to correct deformities. In addition, spinal implant systems can aid in a healing process from trauma, or assist in degenerative conditions. These types of implants are designed to resist post operative fatigue and failure until bone fusion occurs.
The forces applied to the functional spinal unit include compression, tension, torsion, and shear. The motion associated with a spine is complex since rotations occur in three dimensions. In addition, the spine is a major mechanism for supporting the human upper body. For these reasons, it is important for a spinal fixation unit to provide a high degree of strength and stability.
Bone screws are normally used to attach the spinal fixation apparatus to the spine. Different types of bone screws exist and are used at various points on the spine. Pedicle screws are typically used with instrumentation systems such as a Dynamic Transverse Traction (DTT) unit, the Steffee-VSP system (AcroMed Corporation), or the Isola Instrumentation (AcroMed Corporation, Cleveland, Ohio). Harrington devised the first universally accepted method of internal fixation for the treatment of spinal deformity.
Another spinal fixation system is the TSRH (Texas Scottish Rites Hospital) Spinal System, by Danek Medical, Inc. This system provides temporary stabilization until a solid spinal fusion develops. The TSRH system is used for such conditions as idiopathic scoliosis, neuromuscular scoliosis with associated paralysis or spasticity, spinal fractures, and neoplastic disease. Deficient posterior elements resulting from laminectomy or spina bifida might also call for use of bone screws.
Pedicle screws, hooks, eyebolt assemblies, hex nuts, transverse rods, and cross-links are used in the TSRH system. In the TSRH system, pedicle screws with a "Y" shaped head are fitted into a patient and then fixed to a stabilizing rod that is positioned parallel to the spine. The "Y" shaped head of current TSRH pedicle screws have a mounting grove that helps fix the pedicle screw perpendicular to the stabilization rod. Art eyebolt system holds the pedicle screw head against the retaining rod.
Unfortunately, this system requires the surgeon to forcibly bend the retaining rod to conform to the lordotic (concave) or kyphotic (convex) curves in the surgical area. It is important to avoid excessive bending and rebending of these rods because fatigue resistance decreases as bending increases, leading to a more likely rod failure. Further, the method of bending the rod to fit precisely against the inserted pedicle screws is time consuming and difficult for the surgeon. Eyebolts can also score these rods, leading to earlier rod failure.
Some components have been used between the rod and the bone screw to provide for angular rotation of a pedicle or bone screw. For example, one component of the Danek TSRH system is a washer having radial splines placed between the bone screw and the retaining rod. Radial splines on the bone screw head mesh with the washer and provide a rotational adjustment for the bone screw. Thus, this system prevents the surgeon from having to bend the retaining rod to accommodate a bone screw that is not positioned exactly perpendicular to the rod. However, the surgeon must still bend the rod in the vertical position because the radial splines of the washer and the radial grooves in the bone screw head will not mesh if they are positioned at different vertical heights.
Thus, the surgeon is still required to spend time bending the rod in the vertical direction to overcome this limitation. Further, the surgeon is normally working in a very confined area, and previous spinal fixation systems do not provide a way for easily guiding the bone screw into proper alignment with the rod. Thus, a need exists for a more versatile spinal fixation system.